Most silicon integrated circuits are formed in a silicon layer that is epitaxially grown on a silicon substrate, since such epitaxial material can have much higher crystalline quality than bulk silicon. On the other hand, commercial gallium arsenide (GaAs) integrated circuits currently typically are made in bulk material, using metal-semiconductor field effect transistor (MESFET) technology, even though similar devices made in epitaxial GaAs, typically grown by molecular beam epitaxy (MBE), are superior in performance to those made in bulk material. An important reason for the current use of bulk GaAs for most commercial circuits is the frequent presence of so-called "oval defects" on MBE-grown GaAs.
Oval defects, so-called because of their appearance, generally are hillocks or (111) faceted growth around nucleation centers with major axis along &lt;110&gt;. Their typical size ranges from about 5 to 20 .mu.m in length, and about 5 to 10 .mu.m in width. Their areal density typically varies from about 10.sup.2 to about 10.sup.5 cm.sup.-2, depending on the growth rate, epitaxial layer thickness, growth conditions and cleanliness of the substrate and the MBE system. Although not all types of oval defects necessarily degrade device performance, at least some of them, if present in the active region (e.g., the gate region of an FET) do degrade the performance of devices. The detrimental effect on device performance of oval defects can be expected to increase as device design rules become smaller. Thus, for MBE to be a production technique for producing epitaxial material for GaAs (and other III-V materials such as InP) high speed devices and large scale integrated circuits, oval defects must be eliminated.
The origin of oval defects in MBE-GaAs has been extensively studied in many laboratories. Fujiwara et al., Journal of Crystal Growth, Vol. 80, pg. 104, have classified oval defects as .alpha. and .beta. types. The .alpha.-type defects lack microscopic core particulates, whereas .beta.-type defects, which have microscopic core particulates, are due to particulates which land on the substrate either during preparation, loading, transferring, or growth. On a carefully prepared GaAs substrate, .alpha.-type oval defects, which are related to Ga "spitting" and/or the presence of Ga.sub.2 O.sub.3, are the dominant oval defects.
Ga spitting is the result of condensation of Ga atoms at or near the relatively cool lip of the MBE Ga crucible. The condensing Ga atoms form drops which subsquently roll back into the molten Ga charge in the hotter bottom part of the crucible. This causes a violent boiling reaction, resulting in ejection of small Ga droplets from the crucible. Many of these droplets will land on the growth surface, resulting in formation of the oval defects. The presence of Ga.sub.2 O.sub.3 in the Ga charge also can result in the formation of oval defects, due to the evaporation and subsequent landing of the oxides on the growth surface. The oxide can be present in the Ga crucible due to, inter alia, improper outgassing of the crucible, the presence of water vapors in the MBE system, or a leak in the system.
Both of the above mechanisms are continuous processes whose rate typically increases with increasing Ga crucible temperature. As a result, the density of oval defects typically increases with both growth rate and growth time.
A number of techniques have been reported that are said to avoid or reduce oval defects. However, such defects continue to remain a problem. For instance, D. G. Schlom et al., Journal of Vacuum Science and Technology, Vol. B7(2), pg. 296, remark that, ". . . while several groups have reported the elimination of oval defects, oval defects remain a problem."
Among the remedies proposed to eliminate the oval defect problem are the following: use of a sapphire crucible instead of the frequently used pyrolytic boron nitride (pBN) crucible (D. G. Schlom et al., op. cit.); a modified method of substrate cleaning (H. Fronius et al., Japanese Journal of Applied Physics, Vol. 25(2), pg. L137; see also U.S. Pat. No. 4,732,648); placing extra parts into the opening of the Ga crucible to inhibit condensed drops from rolling down to the Ga charge (Japanese patent JP 59-64-594); and re-designing the Ga cell to obtain a positive axial temperature gradient toward the cell orifice to prevent formation of Ga drops.
Other approaches have focused on the elimination of Ga.sub.2 O.sub.3 from the melt. K. Akimoto et al., Journal of Crystal Growth, Vol. 73, pg. 117 have disclosed introduction of H.sub.2 into an MBE system to reduce oxides; U.S. Pat. No. 4,426,237 and G. D. Pettit et al., Journal of Vacuum Science and Technology, Vol. B(2), pg. 241, disclose addition of aluminum and/or magnesium to the Ga melt for the same purpose; and Y. G. Chai et al., Applied Physics Letters, Vol. 38(10), pg. 796, disclose that recharging Ga in a clean crucible with fresh solid Ga, whenever the system is opened to the atmosphere, suppresses Ga oxidation.
With the above referred to Mg doping, complete elimination of oval defects was reported. However, the resulting GaAs layers were p-type doped to about 10.sup.19 cm.sup.-3, and thus, for obvious reasons, the method is not generally useful. None of the other prior art methods can typically completely eliminate oval defects. Thus, in view of the technological importance of developing the ability to grow epitaxial GaAs (and other compound semiconductors such as InP) layers that are essentially free of oval defects, a technique for producing such essentially oval defect-free layers that is simple, reproducible, economical, and free of undesirable side effects such as unwanted doping would be highly desirable. This application discloses such a technique.